Device and method for isolating a short-circuited integrated circuit (IC) from other ICs on a semiconductor wafer

ABSTRACT

A circuit for isolating a short-circuited integrated circuit (IC) formed on the surface of a semiconductor wafer from other ICs formed on the wafer that are interconnected with the short-circuited IC includes control circuitry within the short-circuited IC for sensing the short circuit. The control circuitry may sense the short circuit in a variety of ways, including sensing excessive current drawn by the short-circuited IC, and sensing an abnormally low or high voltage within the short-circuited IC. Switching circuitry also within the short-circuited IC selectively isolates the short-circuited IC from the other ICs on the wafer in response to the control circuitry sensing the short circuit. As a result, if the wafer is under probe test, for example, testing can continue uninterrupted on the other ICs while the short-circuited IC is isolated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/944,509,filed Aug. 30, 2001, now U.S. Pat. No. 6,452,415 B1, issued Sep. 17,2002, which is a continuation of application Ser. No. 09/083,819, filedMay 22, 1998, now U.S. Pat. No. 6,313,658, issued Nov. 6, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to integrated circuits (ICs)fabricated on semiconductor wafers and, more specifically, to devicesand methods for isolating a short-circuited IC from other ICs on asemiconductor wafer so that, for example, probe testing may proceed onthe other ICs on the wafer despite the presence of the short

2. State of the Art

As shown in FIG. 1, integrated circuits (ICs) 10 are small electroniccircuits formed on the surface of a wafer 12 of semiconductor material,such as silicon, in an IC manufacturing process referred to as“fabrication.” Once fabricated, ICs 10 are electronically robed toevaluate a variety of their electronic characteristics. Probingtypically involves positioning needle-like probes (not shown) onto bondpads 14 on the surfaces of ICs 10 to test the ICs 10 using variouselectronic signals supplied through the probes. As described in U.S.Pat. Nos. 5,059,899 and 5,214,657 to Farnworth et al., in some cases,ICs 10 are tested using test probes that contact probe pads 16positioned on the surface of a semiconductor wafer 12 rather than, or inaddition to, contacting bond pads 14 on the ICs 10.

Sometimes shorts develop in some of the ICs 10 on a semiconductor wafer12 as a result of fabrication errors. These shorts can interfere withthe probe testing described above in a variety of ways. For example, insome instances, a supply voltage V_(CC,) provided to ICs 10 on a wafer12 though probes contacting bond pads 14 on the ICs 10 or probe pads 16on the wafer 12, may be shorted to ground through one of the ICs 10. Asa result, over-current protection circuitry, such as a fuse, present intesting equipment that provides the supply voltage V_(CC) to the probes,will likely “trip” the equipment off-line, causing a brief butsignificant delay in the manufacturing of ICs 10 while the equipment isreset. In addition, such a V_(CC)-to-ground short in an IC 10 may makethe entire wafer 12 untestable until the IC 10 with the short isidentified and either repaired or disconnected, which involves aseparate manual process that can cause additional delays in themanufacturing process.

In other instances, a test signal V_(TEST) supplied to a group of ICs 10on a semiconductor wafer 12 through a probe pad 16 on the wafer 12 maybe distorted for all of the ICs 10 in the group by, for example, aV_(TEST)-to-ground or a V_(TEST)-to-V_(CC) short in one of the ICs 10 inthe group. This distortion may interfere with probe testing of all ofthe ICs 10 in the group, and may require that the IC 10 with the shortbe manually identified and repaired or disconnected before the ICs 10 inthe group can be successfully probe tested.

Therefore, there is a need in the art for a device and method forisolating a short-circuited IC on a semiconductor wafer from other ICson the wafer. Preferably, such a device and method should isolate ashort-circuited IC before the IC interferes with probe testing of otherICs so the probe testing can continue uninterrupted.

BRIEF SUMMARY OF THE INVENTION

An inventive device for isolating a short-circuited integrated circuit(IC) from other ICs formed on the surface of a semiconductor wafer thatare interconnected with the short-circuited IC includes controlcircuitry within the short-circuited IC for sensing the short circuit.The control circuitry may sense the short circuit in a variety of ways,including sensing excessive current drawn by the short-circuited IC, andsensing an abnormally low or high voltage within the short-circuited IC.Switching circuitry also within the short-circuited IC selectivelyisolates the short-circuited IC from the other ICs on the wafer inresponse to the control circuitry sensing the short circuit. As aresult, if the wafer is under probe test, for example, testing cancontinue uninterrupted on the other ICs while the short-circuited IC isisolated.

Further embodiments of the present invention are directed to an ICincluding the control and switching circuitry described above, asemiconductor wafer including many of these ICs, and an electronicsystem, such as a computer system, including at least one of these ICs.

In an inventive method for testing ICs formed on the surface of asemiconductor wafer, control circuitry is provided in the ICs forsensing shorts in the ICs. The ICs are then tested, and if the controlcircuitry in one of the ICs senses a short, the short-circuiting IC isautomatically switchably isolated from the other ICs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art top view of a conventional semiconductor wafershowing interconnected integrated circuits (ICs) formed on the surfaceof the wafer;

FIG. 2 is a block diagram and a schematic of an IC including circuitryfor isolating a short-circuiting circuit internal to the IC from asupply voltage V_(CC) bond pad on the IC in accordance with the presentinvention;

FIG. 3 is a top view of a semiconductor wafer including interconnectedICs formed on its surface that are identical to the IC shown in FIG. 2;

FIG. 4 is a block diagram of an electronic system including the IC ofFIG. 2;

FIG. 5 is a block diagram and a schematic of an IC including analternative embodiment of circuitry for isolating a short-circuitingcircuit internal to the IC from a supply voltage V_(CC) bond pad on theIC in accordance with the present invention;

FIG. 6 is a block diagram and a schematic of another alternativeembodiment of circuitry for isolating a short-circuiting circuitinternal to an IC from a supply voltage V_(CC) bond pad on the IC inaccordance with the present invention; and

FIG. 7 is a block diagram and a schematic of still another alternativeembodiment of circuitry for isolating a short-circuiting circuitinternal to an IC from a supply voltage V_(CC) bond pad on the IC inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 2, an integrated circuit (IC) 20 in accordance with thepresent invention includes control circuitry 22 for sensing a short in acircuit 24 internal to the IC 20 and switching circuitry 26 forisolating the internal circuit 24 from a supply voltage V_(CC) bond pad28 on the IC 20 in response to the control circuitry 22 sensing theshort. By isolating the short-circuited internal circuit 24 from thesupply voltage V_(CC) bond pad 28, the present invention can prevent theshort from “tripping” probe test equipment (not shown) supplying thesupply voltage V_(CC) to a semiconductor wafer (not shown) during probetesting of the wafer.

It should be understood that the IC may comprise any IC, including, forexample, a Dynamic Random Access Memory (DRAM) IC and a Static RAM(SRAM) IC. It should also be understood that although the controlcircuitry and switching circuitry will be described with respect tospecific circuitry, the present invention includes within its scope anycontrol circuitry and any switching circuitry capable of performing thefunctions as described. Also, although the control circuitry will bedescribed as sensing the short in the internal circuit by sensing excesscurrent drawn by the internal circuit, the control circuitry may insteadbe constructed to sense abnormally high or low voltages within theinternal circuit indicative of a short circuit. Further, it should beunderstood that while the present invention is considered mostapplicable to probe testing, its applicability is not limited to probetesting. In addition, it should be understood that the present inventioncan be used to isolate short-circuiting internal circuitry of an IC froma wide variety of circuitry external to the IC, and thus is not limitedto isolating internal circuitry from a supply voltage V_(CC) bond pad.

Under normal probe testing conditions of the IC 20, when a short circuitdoes not exist in the internal circuit 24, a ground voltage V_(SS)applied at the gate of a switching PMOS transistor 32 through a largeresistance device 34 (e.g., more than 1 MΩ) turns the switching PMOStransistor 32 on. The ground voltage V_(SS) is also applied at the inputof an inverter 36, which then outputs a high voltage at the gate of ahysteresis PMOS transistor 38, causing the hysteresis PMOS transistor 38to be off. Because the switching PMOS transistor 32 is on, the supplyvoltage V_(CC) applied at the bond pad 28 causes a current I to flowthrough a sensing resistance device 40 and the switching PMOS transistor32 to the internal circuit 24.

The amount of resistance R of the sensing resistance device 40 isselected so that, under normal probe testing conditions, the current Idrawn by the internal circuit 24 causes a voltage drop V (equal to I×R)across the sensing resistance device 40 that is less than the thresholdvoltage |V_(T)| of a sensing PMOS transistor 42. As a result, thesensing PMOS transistor 42 is off.

When a short circuit (e.g., a short circuit to ground) does exist in theinternal circuit 24, the internal circuit 24 rapidly draws excessivecurrent I through the sensing resistance device 40, causing the voltagedrop V across the sensing resistance device 40 to exceed the thresholdvoltage |V_(T)| of the sensing PMOS transistor 42. As a result, thesensing PMOS transistor 42 turns on, thereby applying the supply voltageV_(CC) at the gate of the switching PMOS transistor 32 and at the inputof the inverter 36. Application of the supply voltage V_(CC) at theinput of the inverter 36 causes the inverter 36 to output a low voltageat the gate of the hysteresis PMOS transistor 38, thereby turning thehysteresis PMOS transistor 38 on and reinforcing application of thesupply voltage V_(CC) at the gate of the switching PMOS transistor 32.This causes the switching PMOS transistor 32 to turn off, therebyinterrupting the excessive current flow I and isolating theshort-circuited internal circuit 24 from the bond pad 28.

Because the current flow I is interrupted, the voltage drop V across thesensing resistance device 40 drops to zero, causing the sensing PMOStransistor 42 to turn off. Despite this, the switching PMOS transistor32 remains off, because feedback of the supply voltage V_(CC) from thedrain of the hysteresis PMOS transistor 38 to the input of the inverter36 causes the inverter 36 to continue to output a low voltage at thegate of the hysteresis PMOS transistor 38, thereby causing thehysteresis PMOS transistor 38 to remain on and to continue to apply thesupply voltage V_(CC) to the gate of the switching PMOS transistor 32.The IC 20 remains in this state, with the short-circuited internalcircuit 24 isolated from the bond pad 28, and hence from other ICs undertest, by the switching PMOS transistor 32 until the supply voltageV_(CC) is no longer applied to the bond pad 28, at which point thecontrol circuitry 22 is reset.

As shown in FIG. 3, multiple ICs 20 are formed and interconnected on thesurface of a semiconductor wafer 50 in accordance with the presentinvention. As shown in FIG. 4, an electronic system 52, such as acomputer system, includes an input device 54, an output device 56, aprocessor 58, and a memory device 60 incorporating the IC 20 of FIGS. 2and 3.

As shown in FIG. 5, an IC 70 in accordance with an alternativeembodiment of the present invention includes a fuse 72 for sensing ashort in a circuit 74 internal to the IC 70 and for isolating theinternal circuit 74 from a supply voltage V_(CC) bond pad 78 on the IC70 when excessive current is drawn by the short.

As shown in FIG. 6 in another alternative embodiment of the presentinvention, an IC 80 includes control circuitry 82 for sensing a short ina circuit 84 internal to the IC 80 and switching circuitry 86 forisolating the internal circuit 84 from a supply voltage V_(CC) bond pad88 on the IC 80 in response to the control circuitry 82 sensing theshort. By isolating the short-circuited internal circuit 84 from thesupply voltage V_(CC) bond pad 88, the present invention can prevent theshort from “tripping” probe test equipment (not shown) supplying thesupply voltage V_(CC) to a semiconductor wafer (not shown) during probetesting of the wafer.

Under normal probe testing conditions of the IC 80, when a short circuitdoes not exist in the internal circuit 84, a series of biasing resistors90, 92, and 94 biases the base 96 of a switching bipolar junctiontransistor (BJT) 98 at a voltage intermediate the supply voltage V_(CC)and a ground voltage V_(SS) so that the BJT 98 is on. A voltage takenfrom between the biasing resistors 92 and 94 and applied at the input ofan inverter 100 causes the inverter 100 to output a high voltage to thegate of a hysteresis PMOS transistor 102, causing the hysteresis PMOStransistor 102 to be off. Because the switching BJT 98 is on, the supplyvoltage V_(CC) applied at the bond pad 88 causes a current I to flowthrough a resistor 104 and the BJT 98 to the internal circuit 84.

The amount of resistance R of the resistor 104 is selected so that,under normal probe testing conditions, the current I drawn by theinternal circuit 84 causes a voltage drop V (equal to I×R) across theresistor 104 that is less than the threshold voltage |V_(T)| of asensing PMOS transistor 106. As a result, the sensing PMOS transistor106 is off.

When a short circuit (e.g., a short circuit to ground) does exist in theinternal circuit 84, the internal circuit 84 rapidly draws excessivecurrent I through the resistor 104, causing the voltage drop V acrossthe resistor 104 to exceed the threshold voltage |V_(T)| of the sensingPMOS transistor 106. As a result, the sensing PMOS transistor 106 turnson, thereby applying the supply voltage V_(CC) at the base of theswitching BJT 98 and raising the voltage applied at the input to theinverter 100. The rising voltage at the input of the inverter 100 causesthe inverter 100 to output a low voltage at the gate of the hysteresisPMOS transistor 102, thereby turning the hysteresis PMOS transistor 102on and reinforcing application of the supply voltage V_(CC) at the base96 of the switching BJT 98. This causes the switching BJT 98 to turnoff, thereby interrupting the excessive current flow I and isolating theshort-circuited internal circuit 84 from the bond pad 88.

Because the current flow I is interrupted, the voltage drop V across theresistor 104 drops to zero, causing the sensing PMOS transistor 106 toturn off. Despite this, the switching BJT 98 remains off, because theraised voltage at the input to the inverter 100 causes the inverter 100to keep the hysteresis PMOS transistor 102 on, allowing the hysteresisPMOS transistor 102 to continue to apply the supply voltage V_(CC) tothe base 96 of the BJT 98. The IC 80 remains in this state, with theshort-circuited internal circuit 84 isolated from the bond pad 88, andhence from other ICs under test, by the switching BJT 98 until thesupply voltage V_(CC) is no longer applied to the bond pad 88, at whichpoint the control circuitry 82 is reset.

It should be understood that the switching BJT 98 may be implemented onthe IC 80, as is shown in FIG. 6, in a BiCMOS configuration or,alternatively, may be implemented on a probe card contacting the bondpad 88 or between ICs on a semiconductor wafer.

As shown in FIG. 7 in still another alternative embodiment of thepresent invention, an IC 110 includes control circuitry 112 for sensinga short in a circuit 114 internal to the IC 110 and a micro-relay 116for isolating the internal circuit 114 from a supply voltage V_(CC) bondpad 118 on the IC 110 in response to the control circuitry 112 sensingthe short. By isolating the short-circuited internal circuit 114 fromthe supply voltage V_(CC) bond pad 118, the present invention canprevent the short from “tripping” probe test equipment (not shown)supplying the supply voltage V_(CC) to a semiconductor wafer (not shown)during probe testing of the wafer.

Under normal probe testing conditions of the IC 110, when a shortcircuit does not exist in the internal circuit 114, the controlcircuitry 112 senses no short in the internal circuit 114, so it causesthe micro-relay 116 to close and allow a current I to flow to theinternal circuit 114.

When a short circuit (e.g., a short circuit to ground) does exist in theinternal circuit 114, the internal circuit 114 rapidly draws excessivecurrent I. The control circuitry 112 detects this excessive current Iand, as a result, causes the micro-relay 116 to open, thereby isolatingthe internal circuit 114 from the bond pad 118. The control circuitry112 remains in this state until reset by the voltage at the bond pad 118dropping to zero and then rising again to the supply voltage V_(CC).

It should be understood that the micro-relay may be created usingsilicon micro-machining techniques, and may comprise a capacitively orinductively controlled relay.

Although the present invention has been described with reference toparticular embodiments, the invention is not limited to these describedembodiments. For example, while the various steps of operating theinventive device, and hence the various steps of the inventive method,have been described as occurring in a particular order, it will beunderstood that these steps need not necessarily occur in the describedorder to fall within the scope of the present invention. Thus, theinvention is limited only by the appended claims, which include withintheir scope all equivalent devices and methods that operate according tothe principles of the invention as described.

What is claimed is:
 1. An integrated circuit having an internal circuitand an external terminal for communicating between the internal circuitand circuitry external to the integrated circuit, the external terminalfor receiving a supply voltage from the circuitry external to theintegrated circuit, comprising: a device for isolating the internalcircuit from the external terminal, the device comprising: controlcircuitry for sensing a short in the internal circuit and outputting acontrol signal in response thereto, the control circuitry havingcircuitry for sensing current drawn by the internal circuit exceeding apredetermined threshold and for outputting the control signal inresponse thereto, the control circuitry comprising: a first PMOStransistor having a source coupled to a supply terminal, a gate coupledto an output terminal, and a drain coupled to a control signal terminal;a first resistive device coupled between the supply terminal and theoutput terminal; a second resistive device coupled between the controlsignal terminal and a reference voltage bond pad; a second PMOStransistor having a source coupled to the supply terminal, a gate, and adrain coupled to the control signal terminal; an inverter coupledbetween the control signal terminal and the gate of the second PMOStransistor; and circuitry for sensing a voltage applied to the internalcircuit; and switching circuitry coupled to the control circuitry forisolating the internal circuit from the external terminal in response toreceiving the control signal.
 2. The integrated circuit of claim 1,wherein the external terminal comprises a supply voltage bond pad forreceiving the supply voltage from the circuitry external to theintegrated circuit, wherein the supply terminal is coupled to the supplyvoltage bond pad for receiving the supply voltage, the output terminalfor outputting the received supply voltage, and the control signalterminal for outputting the control signal in response to a current flowbetween the supply terminal and the output terminal exceeding thepredetermined threshold.
 3. The integrated circuit of claim 1, whereinthe control circuitry for sensing a voltage further comprises circuitryfor sensing a voltage applied to the internal circuit below thepredetermined threshold and for outputting the control signal inresponse thereto.
 4. The integrated circuit of claim 1, wherein thecontrol circuitry further comprises circuitry for sensing a voltageapplied to the internal circuit above the predetermined threshold andfor outputting the control signal in response thereto.
 5. An integratedcircuit in a semiconductor die comprising: an external communicationsterminal including a supply voltage bond pad; an internal memorycircuit; switching circuitry coupled between the external communicationsterminal and the internal memory circuit for selectively isolating theinternal memory circuit from the external communications terminal inresponse to receiving a control signal; and control circuitry coupled tothe external communications terminal and the switching circuitry forsensing a short in the internal memory circuit and outputting thecontrol signal to the switching circuitry in response thereto, thecontrol circuitry comprising: a first PMOS transistor having a sourcecoupled to a supply terminal, a gate coupled to an output terminal, anda drain coupled to a control signal terminal; a first resistive devicecoupled between the supply terminal and the output terminal; a secondresistive device coupled between the control signal terminal and areference voltage bond pad; a second PMOS transistor having a sourcecoupled to the supply terminal, a gate, and a drain coupled to thecontrol signal terminal; an inverter coupled between the control signalterminal and the gate of the second PMOS transistor; and circuitry forsensing a voltage applied to the internal memory circuit.
 6. Theintegrated circuit of claim 5, wherein the internal memory circuit isselected from a group of circuits comprising a Dynamic Random AccessMemory (DRAM) circuit and a Static RAM (SRAM) circuit.
 7. Asemiconductor wafer having a plurality of integrated circuits in aplurality of semiconductor dice on the semiconductor wafer, eachintegrated circuit of the plurality of integrated circuits comprising:an external communications terminal for connecting to a supply voltage;an internal memory circuit; switching circuitry coupled between theexternal communications terminal and the internal memory circuit forselectively isolating the internal memory circuit from the externalcommunications terminal in response to receiving a control signal; andcontrol circuitry coupled to the external communications terminal andthe switching circuitry for sensing a short in the internal memorycircuit and outputting the control signal to the switching circuitry inresponse thereto, the control circuitry comprising: a first PMOStransistor having a source coupled to a supply terminal, a gate coupledto an output terminal, and a drain coupled to a control signal terminal;a first resistor coupled between the supply terminal and the outputterminal; a second resistor coupled between the control signal terminaland a reference voltage bond pad; a second PMOS transistor having asource coupled to the supply terminal, a gate, and a drain coupled tothe control signal terminal; an inverter coupled between the controlsignal terminal and the gate of the second PMOS transistor; andcircuitry for sensing a voltage applied to the internal circuit.
 8. Amethod for isolating and testing integrated circuits in each of aplurality of semiconductor dice on a semiconductor wafer, the methodcomprising: providing control circuitry within each semiconductor die ofthe plurality of semiconductor dice for sensing a short in an integratedcircuit in each of the plurality of semiconductor dice, the controlcircuitry comprising: a first PMOS transistor having a source coupled toa supply terminal, a gate coupled to an output terminal, and a draincoupled to a control signal terminal; a first resistive device coupledbetween the supply terminal and the output terminal; a second resistivedevice coupled between the control signal terminal and a referencevoltage bond pad; a second PMOS transistor having a source coupled tothe supply terminal, a gate, and a drain coupled to the control signalterminal; an inverter coupled between the control signal terminal andthe gate of the second PMOS transistor; circuitry for sensing a voltageapplied to the internal circuit; and testing the plurality ofsemiconductor dice so that when the control circuitry in one of theplurality of semiconductor dice senses a short, automatically switchingand isolating an integrated circuit having the short in the one of theplurality of semiconductor dice from other integrated circuits in theplurality of semiconductor dice.
 9. The method of claim 8, whereinproviding control circuitry comprises providing control circuitry withineach semiconductor die of the plurality of semiconductor dice forsensing current drawn by the integrated circuit in a semiconductor dieof the plurality of semiconductor dice that exceeds a predeterminedthreshold.
 10. The method of claim 8, wherein providing controlcircuitry comprises providing control circuitry within eachsemiconductor die of the plurality of semiconductor dice for sensing avoltage within the integrated circuit in a semiconductor die of theplurality of semiconductor dice that is below a predetermined threshold.11. The method of claim 8, wherein providing control circuitry comprisesproviding control circuitry within a semiconductor die of the pluralityof semiconductor dice for sensing a voltage within the integratedcircuit in the semiconductor die of the plurality that is above apredetermined threshold.
 12. The method of claim 8, wherein testing theplurality of semiconductor dice comprises probe testing the plurality ofsemiconductor dice.
 13. The method of claim 8, wherein isolating theintegrated circuit having the short in the one of the plurality ofsemiconductor dice from the other integrated circuits of the pluralityof semiconductor dice comprises opening a switch in the integratedcircuit having the short that couples internal circuitry therein to theother integrated circuits in the plurality of semiconductor dice.
 14. Amethod for isolating a short-circuiting integrated circuit of asemiconductor die of a semiconductor wafer from another semiconductordie formed on the semiconductor wafer interconnected with theshort-circuiting integrated circuit of the semiconductor die, the methodcomprising: sensing the short-circuiting integrated circuit of thesemiconductor die using control circuitry, the control circuitrycomprising: a first PMOS transistor having a source coupled to a supplyterminal, a gate coupled to an output terminal, and a drain coupled to acontrol signal terminal; a first resistive device coupled between thesupply terminal and the output terminal; a second resistive devicecoupled between the control signal terminal and a reference voltage bondpad; a second PMOS transistor having a source coupled to the supplyterminal, a gate, and a drain coupled to the control signal terminal; aninverter coupled between the control signal terminal and the gate of thesecond PMOS transistor; circuitry for sensing a voltage applied to theinternal circuit; and in response thereto, automatically switching andisolating the short-circuiting integrated circuit of the semiconductordie from another semiconductor die.
 15. In an integrated circuit of asemiconductor die of a wafer, the integrated circuit having an internalcircuit, the internal circuit having a reference voltage bond pad and anexternal terminal for communication between the internal circuit andcircuitry external to the integrated circuit, a device for isolating theinternal circuit from the external terminal for communication, thedevice comprising: control circuitry for sensing a short in the internalcircuit and outputting a control signal in response thereto, the controlcircuitry including a supply voltage terminal coupled to the externalterminal for receiving a supply voltage, the external terminalcomprising a supply voltage bond pad for receiving the supply voltage,an output terminal for outputting the supply voltage, and a controlsignal terminal for outputting the control signal in response to acurrent flow between the supply voltage terminal and the output terminalexceeding a predetermined threshold, said control circuitry including: afirst PMOS transistor having a source coupled to the supply voltageterminal, a gate coupled to the output terminal, and a drain coupled tothe control signal terminal; a first resistor coupled between the supplyvoltage terminal and the output terminal; a second resistor coupledbetween the control signal terminal and a reference voltage bond pad; asecond PMOS transistor having a source coupled to the supply voltageterminal, a gate, and a drain coupled to the control signal terminal; aninverter coupled between the control signal terminal and the gate of thesecond PMOS transistor; circuitry for sensing a voltage applied to theinternal circuit; and switching circuitry coupled to the controlcircuitry for isolating the internal circuit from the external terminalfor communication in response to receiving the control signal.